2.4.8 Stream Type

A stream is an object that can be used as a source or sink for
characters--either to supply characters for input or to accept them as
output. Many different types can be used this way: markers, buffers,
strings, and functions. Most often, input streams (character sources)
obtain characters from the keyboard, a buffer, or a file, and output
streams (character sinks) send characters to a buffer, such as a
`*Help*' buffer, or to the echo area.

The object nil, in addition to its other meanings, may be used
as a stream. It stands for the value of the variable
standard-input or standard-output. Also, the object
t as a stream specifies input using the minibuffer
(see section 20. Minibuffers) or output in the echo area (see section 38.4 The Echo Area).

Streams have no special printed representation or read syntax, and
print as whatever primitive type they are.